1. Field of the Invention
The present invention relates to a process for producing a surface-emitting laser, and to a process for producing a surface-emitting laser array.
2. Description of the Related Art
Vertical Cavity Surface Emitting Laser (hereinafter, referred to as VCSEL) has been known as one of surface-emitting lasers. In the surface-emitting laser, an active region is sandwiched on both sides thereof by two reflectors to form a resonator in a direction perpendicular to a substrate surface, and light is emitted in the direction perpendicular to the substrate surface. It is important for the surface-emitting laser to control transverse mode oscillation. When the surface-emitting laser is to be applied to communications, a transverse mode output is required to be a single-mode output. Therefore, in the surface-emitting laser, a current confinement structure is provided in an inner portion thereof by selective oxidation to limit a light emitting region of an active layer, thereby realizing a single transverse mode. However, when the single transverse mode is to be realized by only the current confinement structure, it is necessary to reduce a confinement diameter. When the confinement diameter reduces, the light emitting region becomes smaller, and hence it is difficult to obtain larger laser power.
Therefore, various transverse mode control structures except the current confinement structure have been studied. As one of the control structures, a step structure including a dielectric film is formed on a light emission region of a surface-emitting laser described in Japanese Patent Application Laid-Open No. 2006-210429. When the step structure is formed, a reflectance of a peripheral region surrounding a central region is lower than a reflectance of the central region, and hence high-order transverse mode oscillation may be suppressed.
In this specification, not only the step structure for reflectance control but also a step structure and the like for providing a phase difference between light beams passing therethrough by the step structure to control a wavefront of a near-field pattern (NFP) are referred to as a surface relief structure.
By the way, when the surface relief structure is to be formed, it is important to laterally align the surface relief structure and the current confinement structure with each other. That is, in order to efficiently obtain single mode oscillation in a fundamental transverse mode, the surface relief structure for providing a loss difference and the current confinement structure for determining a light mode are desired to be aligned with each other with high precision. For example, when a center axis of the surface relief structure and a center axis of the current confinement structure are shifted from each other, an unintended loss is introduced into an oscillation mode (for example, fundamental transverse mode). However, even when a manufacturing method disclosed in Japanese Patent Application Laid-Open No. 2006-210429 is employed, it is difficult to align the center axis of the current confinement structure and the center axis of the dielectric relief structure with each other. To be specific, in Japanese Patent Application Laid-Open No. 2006-210429, a mesa structure is formed, and then the formation of the dielectric film, photolithography, and etching are repeated to provide the relief structure as a laminate structure of the dielectric film on an upper surface of the mesa structure. Therefore, it is difficult to align a center axis of the mesa structure and the center axis of the dielectric relief structure with each other. As a result, it is difficult to align the center axis of the current confinement structure and the center axis of the dielectric relief structure with each other.